1. Field of the Invention
The present invention generally relates to a piezoelectric transformer and more particularly, to an improved piezoelectric transformer which is made compact and is capable of yielding a large output. The present invention also relates to a power supply circuit employing the piezoelectric transformer and a lighting unit employing the piezoelectric transformer.
2. Description of the Prior Art
Since power density per unit volume of a piezoelectric transformer is larger than that of an electromagnetic transformer, the piezoelectric transformer can be made compact and thin. Furthermore, in the piezoelectric transformer, since power conversion is performed by exciting mechanical resonant vibration based on inverse piezoelectric effect, high conversion efficiency can be obtained. Thus, in recent years, the piezoelectric transformer is used for a switching power supply in order to make a power supply circuit of an electronic appliance compact.
FIG. 24 is a perspective view of a conventional Rosen type piezoelectric transformer 1. The conventional piezoelectric transformer 1 includes a rectangular plate 2 made of piezoelectric material. In FIG. 24, electrodes 3 and 4 are, respectively, formed on opposite faces of a substantially left haft portion of the rectangular plate 2 and act as electrodes for a driving portion (an input side), while an electrode 5 for a generator portion (an output side) is formed on one end face of the rectangular plate 2. In case the rectangular plate 2 is made of piezoelectric ceramic material such as lead zirconate titanate (PZT), the left half portion of the rectangular plate 2 is polarized in a thickness direction as shown by the arrow A. The polarization operation is performed by using the electrodes 3 and 4. The right half portion of the rectangular plate 2 is polarized in a lengthwise direction as shown by the arrow B by using the electrodes 3, 4 and 5.
Here, an AC voltage close to a resonance frequency for exciting mechanical vibration of expansion and contraction in the lengthwise direction of the rectangular plate 2 is applied between the electrodes 3 and 4 by using the electrode 4 as a common electrode. Thus, in the piezoelectric transformer 1, the mechanical vibration of expansion and contraction in the lengthwise direction of the rectangular plate 2 is excited. Electric charge is induced between the electrodes 5 and 4 at the generator portion from this mechanical vibration by piezoelectric effect. In accordance with a ratio of an impedance obtained by the electrodes 5 and 4 for the generator portion to an impedance obtained by the electrodes 3 and 4 for the driving portion, a high voltage to which a voltage applied between the electrodes 3 and 4 for the driving portion is raised is picked up between the electrodes 5 and 4 for the generator portion.
However, in the conventional piezoelectric transformer 1 utilizing mechanical vibration of expansion and contraction in the lengthwise direction of the rectangular plate 2, it is difficult to cause flow of large electric current due to its structure and vibration mode as follows. FIG. 25A is a sectional view of the conventional piezoelectric transformer 1 taken along the line XXVA—XXVA in FIG. 24. If the conventional piezoelectric transformer 1 is subjected to (½)—wavelength vibration of expansion and contraction in the lengthwise direction, FIG. 25B shows displacement distribution in the lengthwise direction at a time point, FIG. 25C shows internal stress distribution in the rectangular plate 2 forming the piezoelectric transformer 1 and FIG. 25D shows electric charge distribution induced on a plane perpendicular to the thickness direction of the rectangular plate 2 by the vibration. Meanwhile, in FIG. 25B, the ordinate axis represents vibratory displacement and has signs “+” and “−” corresponding to rightward displacement and leftward displacement in FIG. 25A, respectively.
Generally, in a Rosen type piezoelectric transformer, electric charge is induced by vibration of lengthwise expansion and contraction and is outputted as electric current in accordance with magnitude of distortion caused by vibration of expansion and contraction and area of an electrode for a generator portion. However, in the conventional piezoelectric transformer 1 shown in FIG. 24, since area of the electrode 5 for the generator portion is small, it is difficult to obtain large electric current. Thus, in order to obtain larger electric current from the conventional piezoelectric transformer 1, amplitude of mechanical vibration should be increased so as to further distort the rectangular plate 2. However, in case the rectangular plate 2 of the piezoelectric transformer 1 is made of piezoelectric ceramic material, mechanical strength of a portion 2a where polarization direction is discontinuous is weaker than that of a portion having continuous polarization direction. As shown in FIGS. 25A and 25C, a portion where large stress is generated during ordinary operation substantially coincides with the portion 2a having discontinuous polarization direction in the Rosen type piezoelectric transformer 1. Hence, if amplitude of mechanical vibration increases upon increase of electric power handled by the piezoelectric transformer 1, large stress is applied to the portion 2a having discontinuous polarization direction and thus, cracks are likely to occur at the portion 2a. Therefore, the piezoelectric transformer employing the rectangular plate could not been used for applications in which large electric current is outputted.
Then, a piezoelectric transformer utilizing a radial extensional vibration mode of a disc has been proposed in, for example, Japanese Patent Laid-Open Publication No. 4-167504 (1992) so as to be used in applications for outputting large electric current. FIG. 26 is a schematic top plan view of a known piezoelectric transformer utilizing the radial extensional vibration mode of the disc, which has been proposed for use in applications for outputting large electric current. FIG. 27A is a sectional view taken along the line XXVIIA—XXVIIA in FIG. 26, while FIGS. 27B and 27C show stress distribution and vibration mode (vibratory displacement distribution) of the known piezoelectric transformer of FIG. 26, respectively. This known piezoelectric transformer utilizes third-order radial extensional vibration mode of a piezoelectric ceramic disc 10. At a central portion of the piezoelectric ceramic disc 10, a plurality of electrodes 14 are laminated in a thickness direction so as to form a high-impedance portion 12. An insulating annular portion 15 having no electrode is formed outside the high-impedance portion 12 and a low-impedance portion 11 in which a plurality of electrodes 13 are laminated in the thickness direction is further formed outside the insulating annular portion 15.
In order to impart piezoelectric property to the low-impedance portion 11 and the high-impedance portion 12, polarization operation is performed in the low-impedance portion 11 and the high-impedance portion 12. In the low-impedance portion 11 and the high-impedance portions 12, polarization directions in neighboring ones of layers partitioned by the respective electrodes in the thickness direction are opposite to each other as shown by the arrows in FIG. 27A. Assuming that the known piezoelectric transformer has electric input terminals a and b and electric output terminals c and d for voltage step-down purpose, the high-impedance portion 12 acts as a driving portion and the low-impedance portion 11 acts as a generator portion. In case an AC voltage is applied to the electric input terminals a and b, third-order radial extensional vibration of the piezoelectric ceramic disc 10 is excited in the known piezoelectric transformer and a step-down voltage can be picked up from the electric output terminals c and d. In the known piezoelectric transformer, since a portion where a large stress is generated during ordinary operation does not coincide with a portion where polarization is discontinuous, cracks are not readily produced even if amplitude of mechanical vibration is increased upon rise of electric power handled by the known piezoelectric transformer. The same applies also to drive utilizing a first-order radial extensional vibration mode of a disc.
However, in the known piezoelectric transformer utilizing the radial extensional vibration mode of the piezoelectric ceramic disc 10, since the high-impedance portion 12 disposed at the central portion of the piezoelectric transformer 10 has a laminated structure as shown in FIG. 27A, electrical connection becomes difficult and thus, an electrical connection structure becomes complicated disadvantageously.